Liquid Polymorphism and Double Criticality in a Lattice Gas Model

We analyze the possible phase diagrams of a simple model for an associating liquid proposed previously. Our two-dimensional lattice model combines oreintati onal ice-like interactions and \"{}Van der Waals\"{} interactions which may be repulsive, and in this case represent a penalty for distortion of hydrogen bonds in the presence of extra molecules. These interactions can be interpreted in terms of two competing distances, but not necessarily soft-core. We present mean -field calculations and an exhaustive simulation study for different parameters which represent relative strength of the bonding interaction to the energy penalty for its distortion. As this ratio decreases, a smooth disappearance of the doubl e criticality occurs. Possible connections to liquid-liquid transitions of molecul ar liquids are suggested

Simple Fluids with Complex Phase Behavior

We find that a system of particles interacting through a simple isotropic potential with a softened core is able to exhibit a rich phase behavior including: a liquid-liquid phase transition in the supercooled phase, as has been suggested for water; a gas-liquid-liquid triple point; a freezing line with anomalous reentrant behavior. The essential ingredient leading to these features resides in that the potential investigated gives origin to two effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur

Ab Initio Molecular Dynamics Simulation of Liquid Ga_xAs_{1-x} Alloys

We report the results of ab initio molecular dynamics simulations of liquid Ga_xAs_{1-x} alloys at five different concentrations, at a temperature of 1600 K, just above the melting point of GaAs. The liquid is predicted to be metallic at all concentrations between x = 0.2 and x = 0.8, with a weak resistivity maximum near x = 0.5, consistent with the Faber-Ziman expression. The electronic density of states is finite at the Fermi energy for all concentrations; there is, however, a significant pseudogap especially in the As-rich samples. The Ga-rich density of states more closely resembles that of a free-electron metal. The partial structure factors show only a weak indication of chemical short-range order. There is also some residue of the covalent bonding found in the solid, which shows up in the bond-angle distribution functions of the liquid state. Finally, the atomic diffusion coefficients at 1600K are calculated to be 2.1 \times 10^{-4} cm^2/sec for Ga ions in Ga_{0.8}As_{0.2} and 1.7 \times 10^{-4} cm^2/sec for As ions in Ga_{0.2}As_{0.8}.Comment: 29 pages, 10 eps figures, accepted for publication in Phys. Rev.

Dynamics of liquid acetonitrile at high frequencies

Quasielastic neutron scattering spectra were measured for pure acetonitrile at 25°C. In the framework of a simple model of translation and rotation, it was found that the short-time self-diffusion coeff. of liq. acetonitrile is similar to the long-time one measured by NMR. As far as the rotational motion is concerned, the characteristic time was found to be close to the typical value of the mol. spinning motion. The complete inelastic and quasielastic spectra may be further used to check the results of mol. dynamics simulations of acetonitrile

Dynamics and spatial correlations of tetrapentylammonium ions in acetonitrile

Quasielastic neutron scattering (QENS) spectra were measured for a 0.43 M solution of n‐tetrapentylammonium bromide in deuterated acetonitrile at 25 °C, 5 °C, and −15 °C. Values of the translational diffusion coefficient of the cations were inferred from these data using a simple model of translation and rotation. These values are significantly higher than the ones obtained by nuclear magnetic resonance (NMR) spin–echo measurements. The difference can be explained by the different time scales covered by QENS and NMR. QENS shows essentially the contribution of a second order electrophoretic effect to the diffusion coefficient whereas NMR encompasses both electrophoretic and relaxation effects. Consequently, the combination of both techniques allows the two effects to be separated. The relaxation contribution to the diffusion coefficient was calculated by brownian dynamics simulation and compared to the experimental results. The solvent‐averaged ion pair potentials used for this computation were simultaneously adjusted to the thermodynamic and to the small‐angle neutron scattering data by means of hypernetted chain (HNC) calculations

Lithium bromide in acetonitrile and water: a neutron scattering study

The structure around lithium ions in solutions of lithium bromide in acetonitrile and water has been studied by neutron diffraction. For this purpose the isotopic first-order difference method has been applied to the lithium ion. For a 0.58 M acetonitrile solution it has been found that the bromide anion enters into the first solvation shell around the lithium ion, whereas in the case of a 1.88 M aqueous solution the first hydration shell of the cation is not disturbed by the anion. The solvation number is approximately 3 in the case of acetonitrile and approximately 4.5 in the case of water

Neutron scattering experiments on nonaqueous electrolyte solutions

In this paper we have shown the usefulness of neutron scattering experiments for the study of time-averaged correlations in electrolyte solutions. Special emphasis was laid on nonaqueous solutions which offer a vast field of research for interesting phenomena. It is hoped that such kind of experiments contribute to a better understanding of how charged particles behave in liquid phase

Spherical momentum distribution of the protons in hexagonal ice from modeling of inelastic neutron scattering data

The spherical momentum distribution of the protons in ice is extracted from a high resolution deep inelastic neutron scattering experiment. Following a recent path integral Car-Parrinello molecular dynamics study, data were successfully interpreted in terms of an anisotropic Gaussian model, with a statistical accuracy comparable to that of the model independent scheme used previously, but providing more detailed information on the three dimensional potential energy surface experienced by the proton. A recently proposed theoretical concept is also employed to directly calculate the mean force from the experimental neutron Compton profile, and to evaluate the accuracy required to unambiguously resolve and extract the effective proton potential from the experimental dat